This invention relates generally to a method of measuring absorption coefficients, and, more particularly to a method of simultaneously measuring low optical bulk and surface absorption in solids.
Knowledge of bulk and surface absorption in an optical material is necessary when determining the materials applicability in certain optical systems. For example, high power lasers presently under development require high power laser windows made of optical materials which possess both low bulk and surface absorption properties. If the improper optical material is utilized in the high power lasers, the final result will be one of the following: (a) catastrophic failure of the window, for example, from the various thermomechanical effects which are different from each material and for each cooling configuration; (b) window fracture due to the thermal stresses from the nonuniform distribution of temperature induced by the laser; (c) spontaneous cleavage which occurs unpredictably; (d) permanent crosshatch distortion; and (e) the most difficult to detect, thermal lensing, which distorts the optical quality of the window but does not permanently damage the physical structure thereof.
Optical absorption is known to depend upon material purity, material preparation and component fabrication techniques. By measuring both the bulk and surface absorption spectrum of a solid important information is obtained concerning the structure and purity content of the solid both of which are of utmost importance in determining a material's use in optical and electronic application. In addition such information can give clues to what steps must be taken to lower the optical absorption coefficient if desired.
Most of the conventional techniques for determining optical absorption measure the combined effect of surface and bulk losses. However, to identify the cause of the absorption, it is necessary to separate the two components and determine the value of each. A few methods have been developed for measuring both surface and bulk absorption, but these techniques require either several samples or special large sample sizes and have encountered numerous problems. For example, with prior techniques, if either one of the surface or bulk absorption was substantially greater than the other, it was virtually impossible to determine the smaller absorption. Likewise, if the surface and bulk absorption were substantially identical, accurate measurements of individual surface and bulk absorptions were virtually impossible. In addition, methods heretofore in existence have been found less than desirable for the determination of the very low absorption coefficients needed in optical materials which are incorporated into high power laser systems. Since in recent years substantial progress has been made in the fabrication of low loss optical materials for the visible and infrared, it is essential that new techniques be provided which are capable of measuring in an economical and reliable fashion bulk absorption in the 1 .times. 10.sup.-4 cm.sup.-1 to 1 .times. 10.sup.-6 cm.sup.-1 range.